I see your point, but when I buy a book that is number X of Y of a series, then that book has its own beginning, middle, and end. It may be a part of an ongoing series, but each work can stand alone (at least from my experience in reading). Your experience will be enriched by reading the whole series, but you can just read volume 2 and be just fine.
However, if an episodic game does not have its own beginning, middle, and end--or if there are not significant game play changes from one entry to another--then I am essentially buying parts of a whole, as opposed to buying full works that are simply part of something bigger.
This is a lazy analogy, but I think it works: you can buy a whole pizza for $8, you can buy a slice of a pizza for $1, but nobody wants to buy a half-slice of pizza for $1.
I disagree about books. Say, I buy a standalone book from a particular universe (Star Wars, or Discworld), then fine, that book should stand on its own merit. But for books in a defined trilogy/set, as in Lord of the Rings to which I allude above, I think it is unreasonable to expect someone to read The Two Towers, and walk away satisfied.
In many ways, the crux here is continuity: Penny-Arcade could be said to be episodic, but they primarily avoid arcs of continuity, so that any particular episode of Penny-Arcade can be enjoyed by itself. But Heroes could ALSO said to be episodic, yet just watching Episode 5, Season 1, is going to be a waste of time. Series of games, and episodic games that are heavier on continuity (Half-Life 2) are going to have to employ a different strategy, and face different issues, than games that are episodic/series in that they are essentially little pieces of a game that stand by themselves (American McGee's Grimm games as of late).
As to the lazy analogy, I think it fails to recognize that most of the time, if a pizza costs 8 dollars, and consists of 8 slices, it will be cheaper for you to buy a pizza, than to buy 8 individual slices in separate transactions. There are transaction costs, and businesses factor those in. Nobody wants to buy a half-slice for a dollar, but most will pay 1.25 for a whole slice for a variety of reasons (from the convenience of not having to carry around a whole pizza, to the convenience of getting the slice NOW instead of waiting for the entire next pizza to come out of the oven).